Okhttp websocket server Shutdown detection

Question

I am developing a nodemcu websocket server android client app using java.i successfully created client and connected to it through a websocket client service.i can detect server failure/closed when sending data.but can't detect it at the time of failure that is if server powered off cant know untill some data is send.how to know the server failure at the time of failure.using okhttp 4.1.0 library.can anyone help

Answer 1

how to know the server failure at the time of failure.using okhttp 4.1.0 library.can anyone help

You can't. It's not possible, but, there are workarounds, see below.

Why isn't it possible? Internally, the internet is packet switched, which means data is first gathered up into packets, and then these packets are sent.

Most of the stuff you do on the web feels like it is 'streams' instead (you send 1 character, and one character arrives on the other side). But that's all based on protocols that are built on top of the packet nature of the internet.

When you have an open connection between 2 computers via the internet, no data is actually being sent, at all. It's not like you have a line reserved. Old telephone networks did work like that: When you dialled somebody, you got a dedicated line, and once the line got interrupted, you'd hear beeps to indicate this.

That is not how the internet works. Those wires and everything in between have no idea that there is an open connection at all. That's just some bits in memory on your computer and on the server which lets them identify certain packets as part of the longer conversation those 2 machines were having, is all.

Thus we arrive at why this isn't possible: Given that no packets are flowing whatsoever until one side actually sends data to the other, it is impossible to tell the difference between 'no data being sent right now' and 'somebody tripped over the power cable in the server park'. That's why you don't get that info until you send something (and the reason you get that is only because when you send something, the protocol dictates that the server sends you back a confirmation of receiving what you sent. If that takes too long, your computer will send it a few more times just in case the packet just got lost somewhere, and will eventually give up and conclude that the server can no longer be reached or crashed or lost power, and only then do you get the IOException).

Workarounds

A simple one is to upgrade your own protocol: Dictate that the server or client (doesn't matter who takes the responsibility to do this) sends a do-nothing message at least once a minute. You can then conclude after not receiving that for 100 seconds or so that the connection is probably dead. You can start a timer for 100 seconds, reset it every time you receive any data whatsoever. If the timer ever runs out? Connection is likely dead.

This is somewhat take on this idea built into the protocol that lets you make connections that feel like streams of data. That protocol is called TCP/IP, and the feature is called KeepAlive.

The problem is, you possibly don't get to dictate the TCP/IP settings for your websocket connection. If you can, you can turn on keepalive (for example in java, you use Socket to make raw TCP/IP connections, and it has a .setSoKeepAlive(true) method. Check the API if you can get at the socket or otherwise scan the docs for 'keepalive' and see if there's anything there.

I bet there won't be, which means you have to use the trick I mentioned above: Update your server code to use a timer to send a 'hello!' 60 seconds after any conversation, and update your client code to give up on the connection once 100 seconds have passed (give it 40 additional seconds; sometimes the internet gets a little backed up or servers get a little busy).